Alternator warning alarm

[shockwave]

Hello wonder if someone can help?
I have have a Jeanneau sun fast 32i. Fitted with a Volvo Penta MD 2020D diesel engine
Recently the warning light and audible alarm suggesting the generator is not charging is coming on about 5 seconds after starting up the engine.
I have replaced the domestic dual battery with a new fully charged one. The fan belt is new and tensioned correctly.
Voltage Reading on both batteries is 13.7 starter and 12.9v domestic new without engine running
With engine running at at 1500 revs voltage on both batteries is 14.2 2v.
I disagreeumption is that the alternator is charging ok and the fault must lie somewhere else??
Comments welcome can you help please? Rob

[panoramix]

Your assumption is right. With 1500 rpm and the voltage rising to 14.2 Volts, something is charging the batteries. And that something must be the alternator.

So yes, the fault must lie somewhere else.

Take out the electric schematics and start tracing the wires and so on.